begin low-level runtime implementation

llrtc/README.md

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+# LLRT: Low Level Runtime
+
+## Why?
+
+The same reason for LLVM compiler-rt.  LLVM generates libgcc symbols, such as
+__divdi3 for 64-bit division on 32-bit platform.  They are not also available.
+We need to ship compiler-rt but it is not Windows ready.
+This subproject aims to provide a small portable subset of compiler-rt.
+Start small and add only the things we really needed.
+Performance is not crucial but should not be terrible.
+Functionality and usefullness should be more important than performance.
+
+## Developer Instructions
+
+LLRT implements some functionalities in compiler-rt in ANSI C.
+The C files are compiled using clang to produce LLVM IR which are shipped.
+The IR files are committed in the repository.
+So, remember to build the IR files commit them after modifying the C files.
+
+## Build Requirement
+
+- Make
+- Clang
+- Python
+